Chromophores acceptor

The theory of resonance transfer of electronic excitation energy between donor and acceptor molecules of suitable spectroscopic properties was first presented by Forster.(7) According to this theory, the rate constant for singlet energy transfer from an excited donor to a chromophore acceptor which may or may not be fluorescent is proportional to r 6, where r is the distance... 

Figure 4.32 Light driven proton pump using a donor-chromophore-acceptor triad and quinone proton transporter across a phospholipid bilayer membrane.

The condition for transfer of singlet excitation energy from a fluorophore (donor) to another chromophore (acceptor) to occur is spectral overlap of the donor s fluorescence emission spectrum with the acceptor s absorption spectrum. The efficiency of energy transfer E is defined in eq 15... 

FIGURE 10.1 Donor-Chromophore-acceptor triad. X = a chalcogenide atom oxygen, sulfur, or selenium. 

Photoinduced energy transfer and energy migration processes have been studied extensively in luminescent polymers for several decades and have applications in sensors, optics, and solar energy conversion [1-7], Energy transfer refers to a photophysical process whereby the excitation energy of an excited luminophore (donor D ) moves to a chromophore (acceptor A ). This process can occur by either... 

There is also a radiative energy transfer mechanism termed the trivial mechanism of energy transfer. It is accomplished through radiative deactivation of a luminophore donor and reabsorption of the emitted photon by a chromophoric acceptor. There is no direct interaction between the excited donor and the ground... 

A reverse PET, namely photoinduced charge injection from an electronically excited chromophore to a semiconductor surface, is typically the initial step in dye-sensitized solar cells (DSSCs) of the Gratzel-type, for example, with a trinuclear ruthenium dye 51 (Figure 20.11a) [69]. Such a PET step, reminiscent of the charge separation in DSSCs or in PS II, has been visualized by Kelvin probe force microscopy on a donor-chromophore-acceptor nanocomposite poly[TPA-Ru (tpy)2] ZnO (tpy, 2,2 6, 2"-terpyridine) 52 prepared from a ZnO-immobilized [Ru (tpy)2] chromophore (Figure 20.11b, TPA, tripheny-lamine) [70]. In 52, both the electron acceptor (ZnO) and the electron donor (TPA) are assembled in a directional manner to assist the charge separation step... 

We have developed a technique for measuring initial formation rates of the products resulting from the amino transfer type of transpeptidation (17,18). To accomplish this, we used a chromophore acceptor which changed its spectral characteristics once an amide bond has been formed. The transpeptidation rate constant, k5, can be determined by analyzing the dependence of the initial rates on the acceptor concentration (Fig.7). 

In light of tire tlieory presented above one can understand tliat tire rate of energy delivery to an acceptor site will be modified tlirough tire influence of nuclear motions on tire mutual orientations and distances between donors and acceptors. One aspect is tire fact tliat ultrafast excitation of tire donor pool can lead to collective motion in tire excited donor wavepacket on tire potential surface of tire excited electronic state. Anotlier type of collective nuclear motion, which can also contribute to such observations, relates to tire low-frequency vibrations of tire matrix stmcture in which tire chromophores are embedded, as for example a protein backbone. In tire latter case tire matrix vibration effectively causes a collective motion of tire chromophores togetlier, witliout direct involvement on tire wavepacket motions of individual cliromophores. For all such reasons, nuclear motions cannot in general be neglected. In tliis connection it is notable tliat observations in protein complexes of low-frequency modes in tlie... 

The conjugated chromophore (color-causing) system can be extended by electron-donor groups such as —NH2 and —OH and by electron-acceptor groups such as —NO2 and —COOH, often used at opposite ends of the molecule. An example is the aromatic compound alizarin [72-48-0], also known since antiquity as the ted dye madder. 

Meisel etal. [18-20] were the first to investigate how the addition of a polyelectrolyte affects photoinduced ET reactions. They found that charge separation was enhanced as a result of the retardation of the back ET when poly(vinyl sulfate) was added to an aqueous reaction system consisting of tris(2,2 -bipyridine)ruthenium(II) chloride (cationic photoactive chromophore) and neutral electron acceptors [21]. More recently, Sassoon and Rabani [22] observed that the addition of polybrene (a polycation) had a significant effect on separating the photoinduced ET products in an aqueous solution containing cir-dicyano-bis(2,2 -bipyridine)ruthenium(II) (photoactive donor) and potassium hexacyano-ferrate(III) (acceptor). These findings are ascribable to the electrostatic potential of the added polyelectrolytes. 

Sassoon and Rabani [79] also prepared a two polymer system in which a chromophore was covalently bound to one polyelectrolyte and a donor or acceptor was electrostatically held by the other polyelectrolyte, and showed that its back ET underwent a similar retardation effect. They employed 26 as a photosensitizer, MV2+ as a mediator, and ferricyanide as an acceptor electrostatically bound to the added polycation (polybrene). 

The microphase structure of amphiphilic polyelectrolytes in aqueous solution provides photoinduced ET with an interesting microenvironment, where a photoactive chromophore and a donor or acceptor can be held apart at different locations. Photoinduced ET in such separated donor-acceptor systems allows an efficient charge separation to be achieved. 

MV2 + or SPV as an acceptor [125, 126], They have concluded that the steric protection of chromophores from the quencher crucially affects the efficiency of the initial charge separation. This conclusion is essentially the same as that reached by Morishima et al. [119, 120],... 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Fig. 8. Examples of some of the donor-acceptor substituted TEEs prepared for the exploration of structure-property relationships in the second- and third-order nonlinear optical effects of fully two-dimensionally-conjugated chromophores. For all compounds, the second hyperpolarizability y [10 esu], measured by third harmonic generation experiments in CHCI3 solution at a laser frequency of either A = 1.9 or 2.1 (second value if shown) pm is given in parentheses. n.o. not obtained...

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